Is cordierite the same as iolite?
Yes: iolite is the trade name for cordierite (GIA), one mineral, hardness 7 to 7.5, density 2.60 to 2.66. No IMA row for iolite. Which name to write, and when.
Yes. Iolite is a trade name for cordierite, GIA says: one mineral, Mg2Al3(Si5AlO18) on the IMA list, hardness 7 to 7.5, density 2.60 to 2.66, and no iolite row by script. Write cordierite on a specimen label, iolite beside it for a cut gem. “Water sapphire” is a misnomer: sapphire is corundum, at least 2.49 g heavier in water.
In short
- The criteria first. Choosing between the names is a question of who reads the label. A mineral label follows the IMA list; a gem description follows trade use. GIA says cordierite is better known by its trade name, iolite, and its iolite page reads “iolite (mineralogists call it cordierite)”. The International Gem Society lists iolite as a variety of cordierite.
- The IMA test, by script. The September 2026 IMA-CNMNC Master List has a cordierite row, Mg2Al3(Si5AlO18), status G, 1813, and no row for iolite or water sapphire. Fifteen row names contain the letters iolite, among them chiolite, sepiolite and tapiolite; none has cordierite’s formula.
- The label rule. Cordierite on a specimen label; iolite for a cut gem, if you wish, as an addition to cordierite, not a replacement. Sekaninaite only if the iron is shown to outweigh the magnesium, indialite only if the crystal is shown to be hexagonal; neither can be told by eye.
- Three sources, three sets of numbers. Hardness agrees at 7 to 7.5 in all three sources. Specific gravity is 2.60 to 2.66 on the Handbook sheet, 2.61 at GIA and 2.53 to 2.78 at the gem society, whose top reaches sekaninaite’s 2.76 to 2.77. GIA’s index range, 1.542 to 1.551, sits inside the sheet’s 1.527 to 1.578. The table prints them side by side.
- Water sapphire, weighed. GIA says the name has been used incorrectly for iolite. Sapphire is corundum: a 20.00 g piece reads 14.97 to 15.12 g in water on the corundum sheet, against 12.31 to 12.48 g for cordierite, at least 2.49 g heavier.
| Name | What it is, and who says so | IMA row? | Hardness | Density or SG; 20.00 g piece in water | Use it for | Wrong for |
|---|---|---|---|---|---|---|
| Cordierite | The mineral species, Mg2Al3(Si5AlO18) (IMA) | Yes, G, 1813 | 7 to 7.5 | 2.60 to 2.66; 12.31 to 12.48 g | Any specimen label | Nothing, if the material is cordierite |
| Iolite | Trade name for cordierite (GIA); a variety of cordierite (IGS) | No | 7 to 7.5 (GIA) | 2.61 (GIA), 12.34 g; 2.53 to 2.78 (IGS), 12.09 to 12.81 g | A cut gem, beside the species name | A specimen label on its own |
| “Water sapphire” | Incorrect name for iolite (GIA) | No | As iolite | As iolite | Nothing | Everything: it is neither water nor sapphire |
| Sapphire | Corundum (GIA) | No; corundum has a row | 9 | 3.98 to 4.10; 14.97 to 15.12 g. GIA 4.00, 15.00 g | Corundum that is not red | Any iolite |
| Tanzanite | Zoisite (GIA); first suspected to be cordierite (GIA) | No; zoisite has a row | 6 to 7 (GIA) | 3.35 (GIA); 14.03 g | Blue-violet zoisite | Any iolite |
| Sekaninaite | The iron analogue; series with cordierite (sheet) | Yes, A, 1967-047 | 7 to 7.5 | 2.76 to 2.77; 12.75 to 12.78 g | Material shown to be iron-dominant | Material not analysed |
| Indialite | Hexagonal, dimorphous with cordierite (sheet) | Yes, G, 1954 | 7 to 7.5 | 2.512 (synthetic); 12.04 g | Material shown to be hexagonal | Gem iolite, without a structure test |
Is iolite a real mineral? What GIA and the IMA list say
Iolite is a real mineral under another name: GIA says cordierite is better known by its trade name, iolite. GIA’s iolite page reads iolite (mineralogists call it cordierite) and gives the mineral as iolite (cordierite), Mg2Al4Si5O18, hardness 7.0 to 7.5, specific gravity 2.61 and refractive index 1.542 to 1.551. Its description page derives iolite from the Greek ios, violet, and names cordierite after the geologist Pierre Cordier. The International Gem Society lists iolite as a variety of cordierite.
The IMA test, by script. The September 2026 IMA list was searched as text for exact row names. Cordierite has a row, Mg2Al3(Si5AlO18), status G, 1813, with the country printed as Germany and a question mark. There is no row for iolite or water sapphire. Fifteen row names contain the letters iolite: chiolite, ferrisepiolite, three ixiolites, four nioboixiolites, orthopinakiolite, pinakiolite, sepiolite, tainiolite and two tapiolites. None is iolite, and none has cordierite’s formula.
So the name names the same thing. A cut violet-blue iolite and a grey cordierite crystal in schist are one species; the trade name adds nothing a test would detect. The species page is cordierite, with its formula, optics and localities.
The label rule: which name to write
The label rule: write cordierite on a specimen label; add iolite for a cut gem if you wish; write sekaninaite or indialite only when a test has shown iron dominance or hexagonal symmetry. Iolite is a name the gem trade uses; it is not on the IMA list, and a specimen label that says only iolite leaves out the species. Labelling practice in general is on how to label and catalogue a mineral collection.
Why the two exceptions need evidence. The IMA list has four rows with cordierite’s formula shape: cordierite and sekaninaite, orthorhombic on their sheets, magnesium- and iron-dominant; indialite and ferroindialite, hexagonal, magnesium- and iron-dominant. Worked from the sheets’ analyses, the iron share is 4.0% and 9.0% in the two cordierites and 85.3% in the sekaninaite. The species page works all six analyses and shows why no source read here gives a cut-off. Neither the iron share nor the symmetry can be read by eye.
Where the rule fails. An old label may carry a historical name; this page decodes none, because no source fetched for it defines them. A stone set in jewellery cannot be weighed alone. And GIA notes that when tanzanite was first discovered, gemmologists initially suspected it might be cordierite: a first identification can be wrong, and the label should record what was tested.
Three sources, three sets of numbers for iolite
Hardness agrees and the other numbers do not, quite. The Handbook sheet, GIA and the International Gem Society each give 7 to 7.5. For specific gravity the sheet gives 2.60 to 2.66, GIA 2.61 and the gem society 2.53 to 2.78; worked as 20 − 20/D for a 20.00 g piece, that is 12.31 to 12.48 g, 12.34 g, and 12.09 to 12.81 g in water. The gem society’s top, 2.78, is above sekaninaite’s sheet density of 2.76 to 2.77, so its range does not separate the two.
Indices. The sheet gives α 1.527 to 1.560, β 1.532 to 1.574 and γ 1.537 to 1.578; GIA gives 1.542 to 1.551, inside that span; the gem society 1.522 to 1.578, with birefringence 0.005 to 0.018. Worked from the matched ends of the sheet ranges, birefringence is 0.010 to 0.018, as the live benitoite page prints. GIA’s facts box prints “Birefringence: +0.045, −0.011”, which has the form of an index tolerance, not a birefringence; this page does not use it.
Cleavage and luminescence. GIA says iolite has distinct cleavage in one direction and only fair toughness; the sheet gives cleavage fair on {100} and poor on {001} and {010}. The gem society lists luminescence as none, and the cordierite sheet prints no luminescence line, by a ligature-aware search in which hemimorphite and willemite are the positive controls.
Is iolite a sapphire? The water-sapphire misnomer, weighed
Iolite is not a sapphire: GIA says strongly pleochroic iolite has been incorrectly called water sapphire, and sapphire is a different mineral, corundum. The corundum sheet gives hardness 9 and density 3.98 to 4.10; GIA’s sapphire page gives corundum, specific gravity 4.00 and refractive index 1.762 to 1.770.
The 2.5 g gap. A 20.00 g piece reads 14.97 to 15.12 g in water as corundum and 12.31 to 12.48 g as cordierite, at least 2.49 g apart on the sheets; at GIA’s single figures, 4.00 and 2.61, the gap is 2.66 g. GIA’s tanzanite, zoisite at 3.35, reads 14.03 g, 1.69 g above GIA’s iolite. The method is on how to measure specific gravity at home; the corundum page has its own separation from red garnet, not repeated here.
What the colour does. GIA’s sentence on the misnomer pairs it with iolite’s pleochroism, blue to violet in one direction and pale yellow to colourless in another, and that the deep blues of some iolites are thought to share sapphire’s iron-titanium cause. A shared cause of colour is not a shared species.
Who each name is wrong for
“Iolite” alone is wrong for a specimen label, a museum or exchange record, or any list that follows the IMA. It is a fair addition for a faceted or cabochon stone, written as cordierite (iolite). “Cordierite” is wrong for nothing that is cordierite; it is the correct species name for the gem too.
“Water sapphire” is wrong for everything: GIA calls it incorrect. “Sekaninaite” and “indialite” are wrong for any piece without an analysis or structure test, however blue or however hexagonal-looking; cordierite’s own sheet calls it pseudohexagonal, with cyclical twins.
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